1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and, more particularly, to a backlight unit for an LCD device.
2. Discussion of the Related Art
In general, an LCD device includes a thin film transistor (TFT) array substrate, a color filer substrate, a liquid crystal display panel, a driving unit for driving the liquid crystal panel, and a backlight unit for supplying light to the liquid crystal panel. The thin film transistor (TFT) array substrate and the color filer substrate face each other. The liquid crystal display panel includes a first substrate, a second substrate, and a liquid crystal layer formed between the first and second substrates.
The TFT array substrate includes a plurality of data lines crossing a plurality of gate lines to form a matrix. The data lines are vertically arranged with a specified interval therebetween. The gate lines are horizontally arranged with a specified interval therebetween. Each crossing of the data line and the gate line defines a pixel region.
The color filter substrate includes red, green and blue color filter layers formed at positions corresponding to the pixel regions. A black matrix is provided on the color filter substrate. The black matrix prevents light leakage between the color filter layers and color interference of light passing through the pixel regions.
A common electrode and a pixel electrode are formed at facing inner surfaces of the color filter substrate and the TFT array substrate, respectively. The pixel electrode is formed on the TFT array substrate to correspond to each pixel region. The common electrode is integrally formed on the entire surface of the color filter substrate.
The backlight unit supplies light to the LCD device, which cannot emit light by itself. When light emitted from the backlight unit propagates through the liquid crystal layer, the light transmittance depends on the arrangement of the liquid crystal molecules in the liquid crystal layer. The light transmittance of the liquid crystal molecules is controlled by applying a voltage difference between the common electrode and the pixel electrode to generate an electric field in the liquid crystal layer therebetween. To display a desired image, the light transmittance of each pixel region can be controlled by controlling the voltage difference applied between the pixel electrode and the common electrode to change the arrangement of liquid crystal molecules in the liquid crystal layer.
The backlight unit requires lamps as light sources. Depending on the position of the lamps, the backlight unit is classified as an edge type backlight unit or a direct type backlight. In the edge type backlight unit, lamps are disposed at one side or both sides of the liquid crystal panel. Light generated from the lamps is projected on the entire screen of the liquid crystal panel by a light guide plate.
The direct type backlight unit has been actively developed to cope with the increase in the size of liquid crystal panels, which is growing to larger than 20 inches. In the direct type backlight unit, a plurality of fluorescent lamps are arranged in a row and irradiate light directly onto the entire surface of the liquid crystal panel. The direct type backlight unit is more efficient in light usage than the edge type backlight unit, and is commonly used to provide high luminance for large-screen LCD devices.
The backlight unit can use Cold Cathode Fluorescent Lamps (CCFL), Hot Cathode Fluorescent Lamps (HCFL), Electro-Luminescence (EL), Light Emitting Diodes (LED), and the like, as light sources. CCFL and the LED lamps are commonly used. However, although the fluorescent lamps can provide high luminance and improve luminance uniformity, they do not readily lend themselves to high color reproduction. In addition, although the LED or the EL lamps are amenable to use for high color reproduction, but they have low luminance and low luminance uniformity compared with the CCFL.